Various types of cameras that image a scene to provide distances to features in the scene and thereby capture three dimensions for the features, and for the scene, are well known in the art. Such cameras, often referred to as three-dimensional (3D) cameras include stereoscopic, triangulation, and time of flight, 3D cameras.
Gated, time of flight 3D cameras comprise a photosensitive surface, hereinafter referred to as a “photosurface” and a shutter for shuttering the camera open and closed. Typically a photosurface comprises a photosensitive surface of a semiconductor device such as Charge Coupled Device (CCD) or a Complementary Metal-Oxide-Silicon (CMOS) light sensitive device, having light sensitive pixels formed on a suitable substrate. A shutter used to shutter the camera open or closed may comprise, by way of example, a gated image intensifier, or a solid state electro-optical or acousto-optical modulator.
Shuttering a camera open or closed is also commonly referred to as “gating” a camera open or closed (hence the name “gated time of flight 3D camera”) and refers to respectively enabling or preventing registration by the camera's photosurface of light collected by the camera. Gating a camera open or closed is also referred to herein as respectively gating open or gating closed its photosurface. Terms “gating on” and “gating off” a camera or a camera's photosurface are also used herein and respectively mean gating open and gating closed the photosurface or camera. “Shuttering” or “gating” a photosurface or camera without modification by the adverb “open” or “closed” refers to an operation of gating on and/or gating off the photosurface or camera.
A period of time during which a camera is gated open is an exposure period during which the camera's photosurface registers light that the camera collects and directs to the camera's photosurface. A photosurface registers light by accumulating and storing charge, hereinafter “photocharge”, which the light generates in the photosurface's pixels. The light generates photocharge by creating electron-hole pairs in the photosurface pixels. Depending on a doping configuration of the photosurface, the accumulated and stored photocharge may be either electrons or holes from the electron-hole pairs.
To image a scene and determine distances from the camera to features in the scene, the scene can be illuminated, for example with a train of light pulses radiated from an appropriate light source. Typically, the radiated light pulses are near infrared (NIR) light pulses. The camera is gated open for an exposure period for each radiated light pulse in the train, following a predetermined delay from a time at which the light pulse is radiated. For a feature imaged on a pixel in the photosurface, light reflected by the feature from each radiated light pulse is registered on a pixel imaging the feature if the reflected light reaches the camera during the exposure period following the light pulse.
Following a last light pulse in the light pulse train, charges accumulated in the pixels of the photosurface during the exposure periods are sensed and converted to voltages. The set of voltages representing the accumulated charges is referred to as a “frame” of the photosurface. Since the time elapsed between radiating a light pulse and the exposure period that follows it is known, a time it took imaged light that is registered by the pixels to travel from the light source to the reflecting features and back to the camera is known. Light registered by the pixels, the speed of light and the time elapsed is used to determine distances to features in the scene.
In some gated 3D cameras, only whether or not light is registered on a pixel of the 3D camera's photosurface, and the time elapsed between light pulses and exposure periods are used to determine distance from the 3D camera to a feature in the scene imaged on the pixel. Because it is unknown when within an exposure period light reflected from a light pulse by the feature is registered by the gated 3D camera's photosurface, accuracy of distance to the feature is typically determined to no better than a distance equal to the speed of light times half a sum of the exposure period duration and light pulse width.
In other gated 3D cameras, an amount of light registered by the pixel during the exposure periods is also used to determine the distance to the feature. The amount of registered light is used to indicate when during the exposure periods light from the feature is registered, and thereby to improve accuracy of distance measurements to the feature. For a given and light pulse width and a given exposure period duration, accuracy of distance measurements provided by 3D gated cameras that use amounts of registered light, may be substantially improved relative to that provided by 3D gated cameras that do not use amounts of registered light. Accuracy of distance measurements resulting from use of amounts of registered light may be less than about one tenth of a distance equal to the speed of light times half a sum of the pulse width and duration of the exposure period. In gated 3D cameras in which an amount of light is used to determine distances to the imaged feature, the amount of light registered on the pixel is generally corrected for reflectivity of the feature, dark current and background light.
Accuracy of measurements made with a gated 3D camera is a function of, among other variables, rise and fall times, jitter and pulse widths of the light pulses, and how fast the camera's shutter can gate the camera open and closed. A shutter that is sufficiently fast to gate a photosurface of a 3D camera so that the camera provides distance measurements having a required accuracy is referred to as a fast shutter. Typically a fast shutter that is used to shutter a gated 3D camera is separate from, and external to the camera's photosurface. Such an external fast shutter, such as by way of example, a gated image intensifier noted above, may be capable of being switched between optically blocking and unblocking states in less than a nanosecond (ns) or a few nanoseconds.
It does not appear to be advantageous to gate a 3D gated camera by electronically turning on and turning off, hereinafter also referred to as “electronically shuttering”, the camera's photosurface. Even relatively fast conventional, electronically shuttered interline CCDs have shuttering speeds that are generally in excess of about 100 nanoseconds. In addition, a processing time equal to about a microsecond is typically required to acquire a frame of the CCD's pixels. As a result of the processing time, the CCD cannot be electronically shuttered on and off at frequencies greater than or equal to about a megacycle. Because of shuttering speeds in excess of about 100 nanoseconds and the limits on gating frequency, conventionally, electronically shuttered interline CCDs are generally too slow for use in a gated 3D camera to determine distances without an external, fast shutter.
It is noted that turning or shuttering on, and turning or shuttering off a photosurface refer respectively to initiating and ending an exposure period of the photosurface and not necessarily to initiating or terminating any other function and/or process of the photosurface. For example, turning on and turning off a photosurface does not necessarily refer to initiating or terminating transfer of photocharge to a readout amplifier.
To illustrate constraints on shuttering speeds for 3D gated cameras, it is noted that light sources used to provide light pulses for a gated 3D camera may not provide sufficient energy in a single light pulse so that enough light is reflected by features in the scene from the light pulse and back to the camera to provide satisfactory distance measurements to the features. As a result, as noted above, to acquire distance measurements, the scene is exposed to a train of many light pulses. The camera is gated open and closed for each light pulse, and light from the light pulse is registered in pixels of the camera. Intensity of the light pulses, and their number in the light pulse train, are determined so that an amount of reflected light registered from all the light pulses in the train is sufficient to provide acceptable distance measurements to features in the scene. As many as a thousand light pulses might be required in a light pulse train so that an amount of reflected light that reaches the camera from the scene is sufficient to provide acceptable distance measurements. To reduce imaging time, and or possible image blur, to an acceptable level, the light pulse repetition rate, and corresponding repetition rate of exposure periods, may advantageously be as high as at least 107 per second and consequently have a repetition period of about 100 ns or less. Furthermore, light pulse widths and durations of exposure periods may be equal to about 30 ns or less. Conventional electronic shuttering of CCDs is generally much too slow to provide these shuttering speeds and repetition rates.